Memory devices are typically provided as internal storage areas in computers. The term memory identifies data storage that comes in the form of integrated circuit chips. In general, memory devices contain an array of memory cells for storing data, and row and column decoder circuits coupled to the array of memory cells for accessing the array of memory cells in response to an external address.
One type of memory is a non-volatile memory known as flash memory. A flash memory is a type of EEPROM (electrically-erasable programmable read-only memory) that can be erased and reprogrammed in blocks. Many modern personal computers (PCs) have their BIOS stored on a flash memory chip so that it can easily be updated if necessary. Such a BIOS is sometimes called a flash BIOS. Flash memory is also popular in wireless electronic devices because it enables the manufacturer to support new communication protocols as they become standardized and to provide the ability to remotely upgrade the device for enhanced features.
A typical flash memory comprises a memory array that includes a large number of memory cells arranged in row and column fashion. Each of the memory cells includes a floating-gate field-effect transistor capable of holding a charge. The cells are usually grouped into blocks. Each of the cells within a block can be electrically programmed on an individual basis by charging the floating gate. The charge can be removed from the floating gate by a block erase operation. The data in a cell is determined by the presence or absence of the charge on the floating gate.
Memory devices are typically formed on semiconductor substrates using semiconductor fabrication methods. The array of memory cells is disposed on the substrate. Isolation regions formed in the substrate within the array, e.g., shallow trench isolation, provide voltage isolation on the memory array by acting to prevent extraneous current flow through the substrate between the memory cells. Various memory devices include passive elements, such as capacitors, and/or active elements, such as transistors, that are formed in the periphery, such as for accessing, programming, and erasing the memory cells, e.g., select circuitry having high voltage pumps, etc. Isolation regions formed in the substrate at the periphery provide isolation at the periphery by acting to prevent extraneous current from flowing through the substrate between the passive and/or active elements and the memory array.
Isolation regions are normally formed by forming a hard mask layer over the substrate, patterning the hard mask to define regions of the substrate for removal, removing the substrate material to form trenches in the substrate, and filling the trenches with isolation material to form the isolation regions. One problem with the fabrication of the trenches is the formation of sharp corners in the substrate near an upper surface of the semiconductor substrate adjacent the trenches. These sharp corners may carry stronger electric fields that may cause problems when later forming active regions on either side of the trench. For example, when forming a transistor adjacent to the trench, a gate insulating oxide layer may be grown over the substrate and over the trench, because of the sharp corners, the gate oxide layer cannot be grown with a uniform thickness because it becomes too thin over the sharp corners. The thin gate oxide layer may break down if subjected to a high electric field.
Sharp top corners also cause a problem when filling the trench with an isolation material, e.g., using a high-density-plasma (HDP) chemical-vapor-deposition (CVD) process. HDP CVD processes subject the structure to plasma that also induces an electric field around the sharp corners that causes a non-uniform deposition process and that may create gaps or voids in the isolation material. Therefore, the sharp corners are often rounded.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for alternatives for rounding the sharp corners that form adjacent isolation trenches.